Cellular communications systems continue to grow in popularity and have become an integral part of both personal and business communications. Cellular telephones allow users to place and receive voice calls most anywhere they travel. Moreover, as cellular telephone technology has increased, so too has the functionality of cellular devices and the different types of devices available to users. For example, many cellular devices now incorporate personal digital assistant (PDA) features such as calendars, address books, task lists, etc. Moreover, such multi-function devices may also allow users to wirelessly send and receive electronic mail (email) messages and access the Internet via a cellular network and/or a wireless local area network (WLAN), for example.
Another function that has also been recently implemented in some cellular devices is satellite positioning capabilities. By way of example, some cellular devices include not only a cellular transceiver, but also a Global Positioning System (GPS) receiver that receives GPS satellite positioning signals and allows the device to determine its present location. This information can then be used with applications running on the device to advantageously provide the user with desired mapping and/or navigational information, all from the convenience of his cell phone.
Generally speaking, in such configurations the cellular transceiver within the device operates based upon a different clock signal than the GPS receiver. Since the cellular transceiver regularly communicates with a cellular network when turned on, the device has access to timing information from the cellular network that can be used to keep the cellular clock within the device very accurately calibrated. As a result, in some implementations the cellular device clock signal is, in turn, used to help calibrate the GPS clock so that it too can maintain a high degree of accuracy. This approach is called frequency aiding.
One exemplary system which implements a frequency aiding technique for a GPS receiver in a cellular phone is set forth in U.S. Pat. No. 6,741,842 to Goldberg et al. This patent is directed to a frequency management scheme for a hybrid cellular/GPS or other device that generates a local clock signal for the communications portion of the device using a crystal oscillator. The oscillator output is corrected by way of an automatic frequency control (AFC) circuit or software to drive the frequency of that clock signal to a higher accuracy. Besides being delivered to the cellular or other communications portion of the hybrid device, the compensated clock signal may also be delivered to a comparator to measure the offset between the cellular oscillator and the GPS oscillator. The error in the cellular oscillator may be measured from the AFC operation in the cellular portion of the device. An undershoot or overshoot in the delta between the two oscillators may thus be deduced to be due to bias in the GPS oscillator, whose value may then be determined. That value may then be used to adjust Doppler search, bandwidth or other GPS receiver characteristics to achieve a better Time to First Fix or other performance characteristics.
While such systems may be advantageous for providing more accurate cellular and GPS reference clock signals when timing signals from a cellular network are available, their accuracy may be less than desirable when such timing signals are unavailable. Accordingly, new clock reference configurations may be desirable in such applications.